How do energy efficiency ratings work?
As with many concepts like this, it has to do with the ratios. A car's efficiency is measured, in the U.S, using miles per gallon. If the number is higher it means the car can go further using less fuel. Let's translate this to energy. Since gasoline is burned to provide energy to move a car, a car's fuel efficiency is basically its energy efficiency.
Energy efficiency ratings work just like miles per gallon. The higher the number, the more efficient it is, or the more work it gets done per unit of energy. From what I know, they're frequently used in heating and cooling systems to measure how efficient an AC is working. The US uses the Energy Star rating to rate efficiency, so the more stars it has, the more efficient it is.
As for units, I'm sure it depends on the appliance being tested. AC's use its cooling capacity compared to the volts or watts it needs. Lightbulbs use either watts, volts, or BTUs (British Thermal Units) per hour, depending on the area.